Is this something you'd like to maintain automatically, or just do once? You remember how to use the console (if the latter)?
Picture.find_each do |pic| pic.update caption: pic.item.caption end
If you want to do it every time you save a new picture, then you can add a similar helper to the Picture model, and call it in a before_save callback:
def cache_caption self.caption = item.caption end
before_save :cache_caption
Walter